The present invention relates to an optically stable resonator for producing a laser beam, as is generally known, for example, from the high energy laser technology for metal machining. In that connection, the published articles from the publication "Applied Optics", 1966, pages 1550-1567 or from the publication "Lasers & Applications", 1985, pages 79-83 should be mentioned as literature of interest. The criterion for differentiation of so-called stable and unstable resonators for producing laser beams is also indicated in this literature.
Most of the so-called unstable resonators operate exclusively with reflective optical elements in the resonator which can be thermally loaded more highly than transmissive optical elements because, on the one hand, the reflection losses are smaller than the adsorption losses of transmissive elements and because, on the other, reflective optical elements can be cooled intensively. In particular, the unstable resonators require no transmissively loaded decoupling window as include the so-called stable resonators because with such unstable resonators the beam can be guided out of the resonator through a bore which, by reason of an appropriate arrangement of transversely directed aerodynamic flows is able to maintain the vacuum prevailing on the inside of the resonator with respect to the outside. Power output ranges of above 1.5 kw can be realized without difficulty in the exiting laser beam, however, the beam quality, i.e., the energy distribution over the beam cross section is by no means homogeneous and is also not constant with respect to time at every place. Particularly disadvantageous with the beam of an unstable resonator is the fact that it has a hollow cylindrical shape, i.e., its energy maximum is not located in the beam center but is distributed circularly shaped along the edge. As a result thereof, the working beam of an unstable resonator cannot be optimally finely focused so that by reason of laws applying to optical waves, a certain limit results for the energy density in a focused laser beam.
A better beam quality, namely, a laser beam with a so-called Gauss energy distribution over the beam cross section is quite realizable with a resonator operating in a stable manner for the laser beam production, which can be focused particularly narrowly. Also, a better constancy of the energy profile of such a laser beam with respect to time can be obtained with a resonator operating in a stable manner. However, the high thermal loads of the decoupling window through which the laser beam must pass, is disadvantageous with this type of resonator. At most, power outputs of the order of magnitude of about 1.5 kw are achievable in the working beam with the hitherto known resonator types of construction of stably operating resonators with acceptance of few higher modes. With a view to the aimed-at Gauss energy distribution inside of the produced laser beam, a Fresnel number of 1 is aimed at for the resonator. Reference is made to the aforementioned literature for the significance of this number and for the determination thereof. A very small Fresnel number of the magnitude near 1 requires, however, a very large structural resonator length. Though the outside shape of the resonator itself ca be kept relatively small by a space-saving folded arrangement of the beam path, the optically effective length of the beam path is very large, and it is difficult to adjust the same in a stable and positionally accurate manner which is particularly important for a high power output yield. The smallest thermal changes inside of the resonator may lead to a misadjustment of the mirrors which can have as a consequence higher losses and a burning-through of the optical elements. In that regard--as mentioned already--in particular the decoupling window is endangered because it cannot be cooled intensively as, for example, a mirror. An enlargement of the decoupling window for reducing the energy density also produces little help because with an increasing size of the decoupling window, respectively, of the beam diameter, the Fresnel number becomes larger, and the beam quality decreases rapidly; i.e., with increasing size of the beam diameter, higher modes occur with otherwise the same conditions which one precisely seeks to avoid. By reason of the presence of higher modes in the exiting laser beam, the beam can be focused less well and thus the high energy density required for a good operating result can no longer be obtained.
It is the object of the present invention to provide a stable resonator of the aforementioned type in such a manner that notwithstanding a high overall energy of the exiting light beam, for example, above 1 kw, the energy density at the decoupling window reaches only values which can be controlled without difficulty and that nonetheless an energy distribution in the exiting light beam can be realized at least approximating the Gauss energy distribution.
The underlying problems are solved according to the present invention in that at least one separate optical element is arranged in the resonator which converges the light beam proceeding in the direction toward the end mirror in such a manner that the beam impinging on the end mirror is considerably smaller in diameter than the diameter of the light beam passing through the decoupling window and in that at least one further optical element is arranged in the resonator or functionally integrated with the end mirror which diverges the light beam passing in the direction to the decoupling window corresponding to the extent of the convergence of the oppositely directed light beam. Owing to the converging and the diverging optical element in the resonator, a dual division of the resonator into an amplifier part and into a mode filter part is undertaken. This permits to design the amplifier part to high Fresnel numbers which enables a reduction of the energy density at the decoupling window even though the overall emerging light power is very high. The mode filter part can be designed to a small Fresnel number within the range of 1 which, owing to the small beam cross section, requires only small resonator structural lengths that can be readily controlled. More particularly, the structural length of the resonator is reduced owing to the reduced beam cross section in the mode filter part. With a reduction of the beam diameter to half its size, the beam length in the mode filter part is reduced to only one-quarter with the same Fresnel number.
These and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, several embodiments in accordance with the present invention.